煮饭 = (to) cook rice
煮粥 = (to) cook porridge/gruel/congee. To be absolutely clear, you can say "cook rice porridge".
I think "porridge" is the most widely understood word (everyone has heard of Goldilocks and the porridge she ate), then gruel (particularly in Britain), and finally congee (which many Americans may not understand, because rice porridge is not a staple in most Americans' diet). By the way, congee is pronounced "\ˈkän-jē\".